1. Field of the Invention
The present invention relates to a focus detection apparatus and a focus detection method.
2. Description of the Related Art
Conventionally, a focus detection method according to a so-called phase difference method is employed for image capturing apparatuses such as digital cameras. In this method, an image shift amount of a pair of image signals that are obtained on the basis of light transmitted through different exit pupil regions of a photographing lens is determined, and a defocus amount is determined based on the image shift amount that is determined. According to this focus detection method, it is necessary to correctly estimate a conversion coefficient that is used to convert an image shift amount to a defocus amount.
Further, when an operator is operating a camera, in order to comfortably feel an automatic focus detection (hereunder, referred to as AF) operation, the AF speed is also important in addition to the AF accuracy. To improve the AF speed, when AF is performed in a defocus state, it is necessary to enter an in-focus state with the least amount of AF operations possible. More specifically, since it is desirable that a detected defocus amount is near to the actual defocus amount, in this respect also it is important to accurately estimate the conversion coefficient.
A focus adjustment apparatus that corrects a conversion coefficient in a calibration mode in order to estimate the conversion coefficient with greater accuracy has been disclosed. According to a method described in Japanese Patent Laid-Open No. 2004-12493, prior to photographing, an operation is performed that corrects a conversion coefficient based on a difference between image shift amounts of two images acquired at a plurality of focusing lens positions and a predetermined defocus amount, and the obtained value is stored in the camera. At the time of photographing, an image shift amount is converted to a defocus amount using the corrected conversion coefficient. In this connection, according to Japanese Patent Laid-Open No. 2004-12493, since a multi-point distance measurement method that has a plurality of distance measurement regions inside a photographing region is adopted, correction of a conversion coefficient is performed for each distance measurement region.
However, there are many cases in which the pair of image signals do not match each other due to vignetting caused by the photographing lens, manufacturing variations, or looseness in the apparatus. Therefore, it is not always possible to accurately determine an image shift amount for correcting a conversion coefficient. In particular, according to the apparatus disclosed in Japanese Patent Laid-Open No. 2004-12493 in which a multi-point distance measurement method is adopted, there are cases in which the degree of matching between two images at the periphery of a photographing region is low and an image shift amount can not be determined, and consequently an accurate conversion coefficient can not be determined.
Japanese Patent Laid-Open No. 2007-333720 discloses technology in which, as a measure to counter vignetting caused by a photographing lens, shading correction is performed by multiplying data of pixels that receive light that has passed through one region of an exit pupil of a photographing lens by data of pixels of lines that are in the vicinity of the former pixels and receive light that has passed through the other region of the exit pupil. However, there is the problem that, in order to perform shading correction, the circuit scale is enlarged and the calculation times increase.